The worldwide introduction of LIE (Long Term Evolution), which is a next-generation mobile communication standard, has already started. Adopting OFDM (Orthogonal Frequency Division Multiplexing) and MIMO (Multiple-Input Multiple-Output) techniques or the like for downlink and SC-FDMA (Single Carrier-Frequency Division Multiple Access) or the like for uplink has enabled LTE to achieve drastic throughput improvement and also to flexibly assign physical channels to time and frequency domain radio resources.
Furthermore, studies have been carried out on an SON (Self Organizing Network) to simplify installation and operation of base stations in recent years. Applying the SON enables automatic optimization of parameters (e.g., cell ID, channel bandwidth, and transmission power) of each base station without a prior field survey or radio zone design that is normally performed by a communication carrier, thus leading to a reduction in the installation and operation costs of base stations.
PTL 1 discloses a means for setting the cell ID of a newly installed base station in such a way that the cell ID not to overlap with the cell ID of a neighboring base station, as an example of SON. More specifically, PTL 1 discloses a means for the newly installed base station to directly acquire a cell ID from the neighboring base station via an X2 interface, a means for acquiring a cell ID on the basis of a radio signal transmitted from the neighboring base station, a means for acquiring a cell ID via a terminal apparatus and a means for directly acquiring a cell ID from a network node that manages cell IDs of base stations, for example. The newly installed base station using one of these means acquires the cell IDs of the neighboring base stations and assigns any cell ID that does not overlap with those cell IDs as the cell ID of the base station.
In the LTE system mentioned above, a cell ID is an important parameter that determines not only the identification number of the cell but also the sequence (hereinafter referred to as “CRS sequence”) that forms a downlink reference signal (hereinafter referred to as “CRS: Cell-specific Reference Signal”), and a frequency resource assignment. The details of the method of generating a CRS sequence and resource allocation method are disclosed in NPL 1, and an overview thereof will be described herein.
First, CRS sequence rl,ns(m) is expressed by following equation 1.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                                r                              l                ,                                  n                  s                                                      ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB                              max                ,                DL                                              -          1                                    [        1        ]            
In equation 1, ns denotes a slot number, l denotes an OFDM symbol number in a slot, and c denotes a pseudonoise sequence. Here, pseudonoise sequence c is defined by a shift register having a sequence length of 31 and expressed by the following equation using variable n.[2]c(n)=(x1(n+NC)+x2(n+NC))mod 2  (Equation 2)[3]x1(n+31)=(x1(n+3)+x1(n))mod 2  (Equation 3)[4]x2(n+31)=(x2(n+3)+x2(n+2)+x2(n+1)+x2(n))mod 2  (Equation 4)
Here, Nc=1600. The shift register expressed by equation 3 is initialized by following equation 5 for each OFDM symbol.[5]x1(0)=,x1(n)=0,n=1,2, . . . ,30  (Equation 5)
Similarly, the shift register expressed by equation 4 is initialized with a value resulting from converting the value of a decimal number into a binary number, the value of a decimal number being obtained by equation 6 below for each OFDM symbol.[6]cinit=210·(7·((ns+1)+l+1)·(2·NIDcell+1)+2·NIDcell+NCP  (Equation 6)
Here, NcellID denotes a cell ID, which can take a value of 0 to 503. NCP denotes a CP (Cyclic Prefix) length and “1” is inputted thereto for a normal CP and “0” is inputted thereto for an extended CP.
Although it may depend on the operation policy of the communication carrier, an extended CP is applied for the purpose of providing coverage for a region with an extremely low population density by using a single base station on a large scale. For this reason, a base station using an extended CP and a base station using a normal CP are hardly used in mixture, whereas a common CP length is often selected for each region. In such a case, the CRS sequence is uniquely determined by a cell ID.
As an example, FIG. 1 shows a normalized cross-correlation of a CRS sequence with cell ID=100 and CRS sequences with other cell IDs under a condition of channel bandwidth 5 MHz, normal CP, ns=0, l=0 (that is, subframe leading CRS). Thus, the CRS sequences are not orthogonal to each other and generally have a cross-correlation on the order of 0.1 to 0.3.
Next, a CRS resource assignment method will be described. FIGS. 2A to 2C shows an example of a CRS resource assignment in one resource block in one subframe, which is a transmission unit of LTE. In FIGS. 2A to 2C, the horizontal axis shows the time domain and the width of one grid on the horizontal axis represents the time length of one OFDM symbol. The vertical axis shows the frequency domain and the width of one grid on the vertical axis represents one subcarrier. Each shaded block indicates the assignment position of a CRS.
CRS resource assignment in the time direction is uniquely determined by the presence or absence of MIMO transmission of the base station and a CP length. FIG. 2 shows an example of a case with no MIMO transmission and with a normal CP. In this case, CRSs are assigned in the first, fifth, eighth and twelfth OFDM symbols.
Regarding the frequency direction, CRSs are assigned every six subcarriers and their positions are determined by a parameter called resources index vshift. That is, resource index vshift shows the frequency assignment for CRSs transmitted by the base station. Resource vshift takes an integer of 0 to 5 and is calculated based on cell ID as shown in following equation 7.[7]vshift=NIDcell mod 6  (Equation 7)
FIG. 2A shows a CRS assignment when vshift=0. CRSs are assigned in a cyclically shifted manner in the frequency direction by vshift using the assignment of vshift=0 shown in FIG. 2A as a reference. That is, FIG. 2B and FIG. 2C show assignments when vshift=2 and when vshift=4, respectively.